Brake mechanisms used in heavy duty automotive applications are generally of the pneumatic type in which air pressure is used to apply the vehicle's brakes. Typically, such brake mechanisms include a drum which rotates concentrically with a wheel to be braked and a brake shoe carried by the frame of the vehicle that may be frictionally engaged with an interior wall of the drum by air pressure.
The brake shoe is moved into and out of engagement with the drum about a crankshaft that is mounted on the vehicle frame for rotation about its axis. The crankshaft is turned about its axis by a slack adjuster that is connected to a pneumatic actuator. The slack adjuster includes an interiorly splined adjustment ring at one end for receiving the crankshaft and is attached at its other end to the pneumatic actuator. Together, the crankshaft and slack adjuster form a lever arm acting about the rotational axis of the crankshaft for pivoting the brake shoe into and out of engagement with the drum in response to movement of the pneumatic actuator.
Typically, the pneumatic actuator includes a cylinder mounted on the vehicle frame and a piston having a limited range of travel within the cylinder. The piston includes an arm that is connected to the slack adjuster. In accordance with the known operation of such actuators, pressurized air introduced into the cylinder moves the piston within the cylinder and, through the piston's connection to the slack adjuster, also moves the brake shoe into engagement with the drum. However, when the pressurized air is released from the cylinder, a spring biases the brake shoe away from the drum and returns the piston to its initial or "resting" position within the cylinder.
When the brake shoe is so biased away from the drum, a small clearance space is defined between the shoe and drum which permits the associated wheel to rotate freely. The amount of clearance space, also referred to as backlash, must be maintained within a specific tolerance to insure proper operation of the brakes. For example, if not enough backlash is present, the brake shoe may continuously drag against the drum leading to a buildup of heat and premature wear of the contacting parts. Alternatively, if too much backlash is present, the distance through which the piston must travel prior to the brake shoe making contact with the drum increases; and this may undesirably delay braking response of the vehicle. Variations in the amount of backlash between different wheels may also result in uneven braking responses of the wheels and thereby destabilize the vehicle during braking.
Accordingly, it is of well-recognized importance to maintain backlash between the brake shoe and drum within a specified tolerance. However, during the course of ordinary vehicle use, the brake shoe and drum tend to wear, thereby increasing the amount of backlash between them. In contrast, if a worn brake shoe is replaced with a new one, backlash may be decreased. Thus, periodic adjustments to the amount of backlash between the brake shoe and drum are required to maintain the brakes in good working order.
The slack adjuster includes an adjusting screw for this purpose. The adjusting screw includes a shaped head portion at one end which extends out of the slack adjuster and a threaded portion at its other end that engages the outer surface of the adjustment ring which receives the crank shaft. When the screw is turned, the crank shaft is rotated, thereby moving the brake shoe toward or away from the drum. Backlash may be set by turning the adjusting screw in an appropriate direction until the amount of clearance space between the brake shoe and drum is within a specified tolerance.
The shaped head portion of the adjusting screw is protected by a retractable collar which is slidably mounted in a chamber formed in the outer surface of the slack adjuster. The locking collar is spring loaded within the chamber to urge the locking collar into engagement with the shaped head portion of the adjusting screw, thereby capturing the head portion of the screw within the collar. A keyway is provided within the slack adjuster chamber to receive a key portion of the collar to prevent the collar and the captured head portion of the adjusting screw from turning. However, when an adjustment is to be made to backlash, the locking collar may be retracted against the spring within its chamber to fully expose the head portion of the adjusting screw. A wrench may be used on the exposed head portion of the adjusting screw to turn the screw to a desired backlash setting. Following removal of the wrench, the locking collar is allowed to return to its original position to prevent any further turning of the screw.
However, even after relatively short periods of vehicle use, rust begins to form on the locking collar and in the chamber within which it is mounted. This condition is often aggravated by road salts and acids used for cleaning trucks. As a result, the locking collar is no longer easily slidable within its chamber. When this happens, it is a common practice to retract the locking collar by pounding it with a hammer to expose the adjusting screw. Although this practice is generally effective for retracting the locking collar, a more serious problem may follow with the locking collar becoming stuck in its retracted position. However, common practices for freeing locking collars stuck in retracted positions, such as pounding on the collar with chisels or attempting to grab the locking collar with vise grips, are often not very effective for freeing the collar and may permanently damage the collar.
Unless the locking collar can be returned to its original position protecting the adjusting screw, the entire slack adjuster must be replaced at a cost of considerable time and expense. Further, if it is necessary to make an emergency brake adjustment where replacement parts are not available and the locking collar becomes permanently stuck in a retracted position, it is possible that the vehicle will be driven without the protective benefits of the locking collar guarding against loosening of the adjusting screw. If the adjusting screw is left unguarded for even a relatively short period of time, vibrations associated with normal brake use may result in a loss of proper braking at the affected wheel.